1. Field of the Invention
The present invention relates generally to impedance matching to amplifiers, and more particularly to an impedance matching circuit and method for matching an output impedance of an amplifier circuit in a wireless communications system to an input impedance of a transmission circuit.
2. State of the Art
Wireless communication systems typically include a chain of amplifier circuit through which a received or modulated signal is passed in series. The output of the amplifier stages is coupled to a load, typically via an impedance matching circuit.
Impedance matching circuits help match the output impedance of the amplifier stages to the impedance of the load. An ideal impedance match provides for the maximum transfer of power from the source to the load. In a wireless communication system, for example, it is typically desirable to maximize the power delivered from a final amplifier circuit or power amplifier to an antenna. The maximum power is transferred from the power amplifier to the antenna when, for a given frequency, the input impedance of the antenna is equal to the conjugate of the output impedance of the power amplifier. When these conditions are satisfied power is delivered with 50% efficiency, that is as much power is transferred to the antenna as is dissipated in the internal impedance of the power amplifier.
Generally, the output impedance of a power amplifier will not be what is needed for maximum power transfer. For example, a typical power amplifier used in a handset of a wireless communication system may have an internal impedance of about 3 ohms, whereas the antenna used in the same handset has an input impedance of about 50 ohms. Typically, a matching network comprising capacitors and inductors is inserted between the power amplifier and the antenna to make the power amplifier output impedance appear to be the complex conjugate of the input impedance of the antenna.
But conventional matching circuits may require non-standard values for L and C to achieve a proper impedance match. Consequently, it generally is not possible to design and build a matching network that precisely matches the impedance of the power amplifier to that of the antenna This problem is exacerbated by the variations in values of the inductors and capacitors due to uncertainties or tolerances in their manufacturing processes. Inductors and capacitors can vary by 10% or more from their specified value. Accordingly, the larger the values of the inductors and capacitors the larger the impact of the manufacturing tolerances on the impedance matching. Thus, implementing a matching network to precisely match impedance of a power amplifier and antenna to achieve maximum power transfer can be a challenge.
Ideally, inductors and capacitors used in a matching circuit would have no resistance and the matching circuit would therefore dissipate little or no power. But, in reality, a matching circuit can dissipate several percent of the power being transferred to the load or antenna. Thus, to minimize dissipation of power it is generally desirable to keep inductors'and capacitors'values as small as possible.
A particular problem with conventional matching networks used in wireless communications systems is that the transceivers or transmitters must often operate at multiple frequencies. For example, a dual band GSM/DCS radiotelephone handset uses the Global System for Mobile Communications (GSM) standard around 900 MHz, and the Digital Communications System (DCS) standard, which is similar to GSM except that it operates around 1800 MHz.
Accordingly, there is a need for a matching circuit and method that can provide equal or substantially equal impedance between an amplifier circuit and a transmission circuit, thereby increasing power efficiency and reducing signal distortion. There is a further need for a matching circuit and method with the ability to match impedance for signals at multiple frequencies. It is further desirable that the matching circuit and method reduce the number or size of components used in the matching circuit, including the length of any transmission line used.